In a fully developed Rayleigh-Bénard turbulent flow,
most of the imposed temperature difference is localized in the thermal boundary layers near the surface of the top and bottom plates, within which heat is transported via conduction [1]}, [2]}, [3]}. The velocity field has the same character: velocity gradient is localized in a thin layer near the plates, which is called viscous boundary layer. Turbulent flow in the central region of the RB cell is approximately homogenous and isotropic[4]}, [5]}, [6]}. As the top and bottom boundary layers contribute the main resistance to heat transfer through the cell and thus dominantly determine the Nusselt number, they deserve special attention. Indeed, nearly all theories in RB convection are in essence boundary-layer (BL) theories. For example, a turbulent BL was assumed in the early marginal stability theory [7]} and also in the models by [8]} and [9]} and by [10]}, [11]}. On the other hand, a Prandtl-Blasius (PB) type laminar BL was assumption in the Grossmann & Lohse (GL) theory [12]}, [13]}, [14]}, [15]}. Therefore, direct characterization of the BL properties is essential for testing and differentiating the various theoretical models, and will also provide insight into the physical nature of turbulent heat transfer.
